Upper-mantle seismic anisotropy from SKS splitting in the South American stable platform: A test of asthenospheric flow models beneath the lithosphere

Abstract

Upper-mantle seismic anisotropy has been extensively used to infer both present and past deformation processes at lithospheric and asthenospheric depths. Analysis of shear-wave splitting (mainly from core-refracted SKS phases) provides information regarding uppermantle anisotropy. We present average measurements of fast-polarization directions at 21 new sites in poorly sampled regions of intraplate South America, such as northern and northeastern Brazil. Despite sparse data coverage for the South American stable platform, consistent orientations are observed over hundreds of kilometers. Over most of the continent, the fast-polarization direction tends to be close to the absolute plate motion direction given by the hotspot reference model HS3-NUVEL-1A. A previous global comparison of the SKS fast-polarization directions with fl ow models of the upper mantle showed relatively poor correlation on the continents, which was interpreted as evidence for a large contribution of "frozen" anisotropy in the lithosphere. For the South American plate, our data indicate that one of the reasons for the poor correlation may have been the relatively coarse model of lithospheric thicknesses. We suggest that improved models of upper-mantle fl ow that are based on more detailed lithospheric thicknesses in South America may help to explain most of the observed anisotropy patterns.

Upper mantle anisotropy of the Borborema Province, NE Brazil: Implications for intra-plate deformation and sub-cratonic asthenospheric flow

“…5 and 7. Observations from Assumpção et al (2011) are consistent with nearby stations from our study, with the exception of station PDCP, Fig. 4.…”

“…Accordingly, just to the south of our study area, in SE Brazil, Heintz et al (2003) observed large splitting delay times (N2.5 s), which they cited as evidence for a sub-lithospheric contribution to their results. Assumpção et al (2006) favoured an asthenospheric flow hypothesis to explain their SKS splitting results in SE Brazil; Assumpção et al (2011) also explained the N-S to NE-SW fast directions at stations RCBR and PDCB (Table 3 and Figs. 5 and 7) in this way.…”

“…We cannot preclude the possibility that this assumption is invalid, since our backazimuthal coverage of earthquakes is insufficient to resolve more complex dipping or multi-layer patterns of anisotropy (e.g., Hartog and Schwartz, 2000;Silver and Savage, 1994). For completeness, we include the results of four stations, CS6B, PDCB, RCBR, and TRSB, from the study of Assumpção et al (2011) in Table 2, and Figs. 5 and 7.…”

“…Ours is the first regional study of mantle anisotropy beneath NE Brazil using teleseismic shear wave splitting, though neighbouring studies do exist: to the south of our study area, in SE Brazil, Heintz et al (2003) and Assumpção et al (2006) observed large splitting delay times (N2.5 s), which they cited as evidence for co-aligned fossil lithospheric fabrics and sublithospheric mantle flow: the two anisotropic layers are postulated to interfere constructively, yielding the large delay times. Assumpção et al (2011) carried out a splitting study of the entire South American stable platform, but that study featured only two stations in the Borborema Province. Studies of local earthquakes in the Borborema Province indicate that the crust records fossil anisotropic fabrics that parallel regional metamorphic trends (e.g., do Nascimento et al, 2002(e.g., do Nascimento et al, , 2004, but splitting delay times for these shallow (≤5 km) events vary considerably between 1 and 28 ms per kilometre of crust sampled.…”

Domains of Archean mantle lithosphere deciphered by seismic anisotropy – inferences from the LAPNET array in northern Fennoscandia

“…1b), allows us to associate it with fossil structure of the mantle lithosphere and not with anisotropy reflecting an olivine orientation due to the present-day flow and/or local circulations in the sub-lithospheric mantle, which is, on top of that, generally considered to be very small beneath cratons (Montagner, 1998;Pedersen et al, 2006). Assumpção et al (2011) explain variations in average splitting parameters beneath the South America cratons by local deflections of the sub-lithospheric flow due to lithosphere thickness variations. On the other hand, Barruol et al (2008) and Wüstefeld et al (2010) found several arguments supporting frozen lithospheric anisotropy in cratonic areas.…”